The present invention relates generally to medical electrical leads and more specifically to electrode leads used in conjunction with implantable pacemakers and implantable defibrillators.
Pacing and defibrillation leads are typically located on or in the human heart. The physician carefully places the leads so that their electrodes are located precisely in desired locations. However, the beating of the human heart tends to dislodge these leads from their desired locations. Therefore, over the years, a wide variety of methods and apparatus designed to retain the leads in their desired locations have been developed.
Lead fixation mechanisms can generally be divided into active fixation and passive fixation. Active fixation devices typically take the form of penetrating barbs, screws, or clamps which actively engage, and typically penetrate heart tissue as part of their retention function. Leads employing active fixation mechanisms include U.S. Pat. No. 3,737,579 issued to Bolduc, U.S. Pat. No. 3,814,104 issued to Irnich et al, U.S. Pat. No. 3,844,292 issued to Bolduc, U.S. Pat. No. 3,974,834 issued to Kane and U.S. Pat. No. 3,999,555 issued to Person. Passive fixation mechanisms are typically less severe, and tend to engage the heart tissue without penetrating it. Pliant tines, located upon the ends of the electrodes are the most commonly used passive fixation device. Such tines are disclosed in U.S. Pat. No. 3,902,501, issued to Citron, et al. Alternative passive fixation mechanisms include leads specifically shaped to brace against cardiac tissue, so that the electrodes will remain in a specific desired location, as well as fixation by means of tissue ingrowth, wedging, and so forth. Leads employing passive fixation mechanisms are also disclosed in U.S. Pat. No. 4,154,247 issued to O' Neill, U.S. Pat. No. 4,149,542 issued to Thoren and U.S. Pat. No. 3,937,225 issued to Schramm.
All of the fixation mechanisms described above have one or more drawbacks. Typically, passive fixation mechanisms are not as reliable in maintaining the leads in their desired locations as active fixation mechanisms, in the absence of trabeculation at the desired electrode location. Active fixation mechanisms typically require deployment of some sharpened member such as a screw or a barb, which adds substantial mechanical complexity, and in some cases risks unwanted perforation of heart tissue or snagging of the fixation devices on venous or valve tissue, during the implantation procedure. Thus, there is still a demand for improved fixation mechanisms, particularly those which do not significantly add to the mechanical complexity of the lead or to the difficulty of the implant procedure, but nonetheless provide reliable fixation at the time of implant.